The present invention relates to a fine carbon fiber which is used as a filler material incorporated into various materials such as metal, resin, and ceramic, to thereby improve electrical conductivity and thermal conductivity, which is used as an electron emission material for producing a field emission display (FED), which is used as a medium for occluding hydrogen, methane, or various other gasses, and which is used as a filler material employed in, for example, materials for improving properties of batteries; and to a method for producing the fine carbon fiber.
The present invention also relates to a battery electrode containing the fine carbon fiber so as to attain improved charge/discharge capacity and exhibit improved strength, the battery electrode being employed as a positive or negative electrode of any of a variety of secondary batteries such as dry batteries, Pb storage batteries, capacitors, and recently developed Li-ion secondary batteries.
In the late 1980""s, studies were conducted on vapor grown carbon fiber (hereinafter abbreviated as xe2x80x9cVGCFxe2x80x9d). VGCF having a diameter of 1,000 nm or less and a length of some tens of xcexcm or less is known to be produced through thermal decomposition of a gas of, for example, hydrocarbon in a vapor phase in the presence of a metallic catalyst.
A variety of processes for producing VGCF are disclosed, including a process in which an organic compound such as benzene, serving as a raw material, and an organo-transition metallic compound such as ferrocene, serving as a catalyst, are introduced into a high-temperature reaction furnace together with a carrier gas, to thereby produce VGCF on a substrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700); a process in which VGCF is produced in a dispersed state (Japanese Patent Application Laid-Open (kokai) No. 60-54998); and a process in which VGCF is grown on a reaction furnace wall (Japanese Patent No. 2778434).
Through the aforementioned processes, there can be produced carbon fiber of relatively small diameter and high aspect ratio which exhibits excellent electrical conductivity and thermal conductivity, and is suitable as a filler material. Therefore, carbon fiber having a diameter of about 10 to about 200 nm and an aspect ratio of about 10 to about 500 is mass-produced, and is used, for example, as an electrically or thermally conductive filler material in electrically conductive resin or as an additive in lead storage batteries.
A characteristic feature of a VGCF fiber resides in its shape and crystal structure. A VGCF fiber has a structure including a very thin hollow space extending along the fiber in its center portion, and a plurality of carbon hexagonal network layers grown around the hollow space so as to form concentric rings.
Iijima, et al., 1991, Nature, 354, 56, have discovered a carbon nano-tube, which is a type of carbon fiber having a diameter smaller than that of VGCF, in soot produced by evaporating a carbon electrode through arc discharge in helium gas. The carbon nano-tube has a diameter of 1 to 30 nm, and is a fine carbon fiber having a structure similar to that of a VGCF fiber; i.e., the tube has a structure including in its center portion a hollow space extending along the fiber, and a plurality of carbon hexagonal network layers grown around the hollow space so as to form concentric rings.
However, the above process for producing the nano-tube through arc discharge has not yet been put into practice, since the process is not suitable for mass production.
Meanwhile, the vapor-growth process may feasibly produce carbon fiber having a high aspect ratio and exhibiting high electrical conductivity, and therefore attempts have been made to improve the vapor-growth process for the production of carbon fiber of smaller diameter. For example, U.S. Pat. No. 4,663,230 discloses a graphitic cylindrical carbon fibril having a diameter of about 3.5 to 70 nm and an aspect ratio of 100 or more. The carbon fibril has a structure in which a plurality of layers of regularly arranged carbon atoms are continuously disposed concentrically about the cylindrical axis of the fibril, and the C-axis of each of the layers is substantially perpendicular to the cylindrical axis. The entirety of the fibril includes no thermal carbon overcoat deposited through thermal decomposition, and has a smooth surface.
In order to attain improvement of VGCF, Japanese Patent Application Laid-Open (kokai) No. 61-70014 discloses a carbon fiber having a diameter of 10 to 500 nm and an aspect ratio of 2 to 30,000, which fiber is produced through a vapor-growth process. According to this publication, a carbon layer obtained through thermal decomposition has a thickness of 20% or less the diameter of the carbon fiber.
When being employed as an electrically conductive filler or a thermally conductive filler, the aforementioned VGCF, carbon nano-tube, or carbon fibril exerts excellent effect by virtue of its structure and shape.
Unlike the case of typical carbon black, VGCF, carbon nano-tube or carbon fibril have a carbon structure of high electrical conductivity and thermal conductivity which has been developed along the axis of a fiber. Therefore, VGCF, carbon nano-tube or carbon fibril have fewer points of contact between particles or between fibers per unit length as compared with the case of carbon black, and thus exhibit low contact resistance. Therefore, VGCF, carbon nano-tube or carbon fibril exert excellent effect in terms of electrical conductivity or similar properties. Furthermore, VGCF, carbon nano-tube or carbon fibril exhibit high strength by virtue of their fibrous shape.
Attempts have been made to provide fine carbon fibers of different structures, and there is disclosed a fine carbon fiber having a herringbone-shaped carbon structure, as well as a fine carbon fiber having a structure including no hollow space in which carbon layers are formed parallel with the axis of the fiber (N. M. Rodriguez et. al., Langmuir., vol 11, pages 3862-3866, 1995).
These attempts have been made in an effort to improve functions of fine carbon fiber, such as occlusion of a gas (e.g., hydrogen).
In view of the foregoing, an object of the present invention is to provide a fine carbon fiber which exhibits improved occlusion of a gas such as hydrogen, and which is used as a filler which, when incorporated into a battery electrode, can improve electrical conductivity, thermal conductivity, and strength.
The present inventors have performed studies in an attempt to obtain a filler which exhibits occlusion of a gas such as hydrogen, and which attains improved electrical conductivity, thermal conductivity, and strength, to thereby improve the structure of VGCF; and as a result have produced a novel fine carbon fiber having an outer diameter of 0.002 to 0.5 xcexcm, an aspect ratio of 10 to 15,000, and a carbon structure differing from that of a conventional fine carbon fiber.
Accordingly, the present invention provides the following embodiments:
1) A vapor grown fine carbon fiber comprising a hollow space along the fiber in its interior, and having a multi-layer structure, an outer diameter of 2 to 500 nm, and an aspect ratio of 10 to 15,000, wherein the fiber comprises a center portion and a peripheral portion, the center portion having a carbon structure different from that of the peripheral portion;
2) The fine carbon fiber according to 1) above, wherein the hollow space of the fiber has a diameter (d0) and the fiber has an outer diameter (d) satisfying the following relation: 0.1dxe2x89xa6d0xe2x89xa60.8d;
3) The fine carbon fiber according to 1) or 2) above, wherein the center portion of the fiber has a diameter (d1), the hollow space of the fiber has a diameter (d0), and the fiber has an outer diameter (d) satisfying the following relations: 1.1d0xe2x89xa6d1 and d1xe2x89xa60.9d;
4) The fine carbon fiber according to any one of 1) through 3) above, wherein the center portion of the fiber contains a herringbone-shaped carbon structure, and the the peripheral portion of the fiber contains a concentric ring carbon structure;
5) The fine carbon fiber according to any one of 1) through 4) above, wherein the hollow space is partially closed;
6) A fine carbon fiber obtained through heat treatment of a fine carbon fiber as recited in any one of 1) through 5) above at about 2,000 to about 3,500xc2x0 C.;
7) The fine carbon fiber according to any one of 1) through 6) above, further comprising boron or a boron compound;
8) The fine carbon fiber according to 7) above, wherein boron (B) is present in an amount of about 0.01 to about 5 mass %, in carbon crystals constituting the carbon fiber;
9) A fine carbon fiber mixture comprising a fine carbon fiber as recited in any one of 1) through 8) above in an amount of about 5 to about 80 vol. % on the basis of the entire carbon fiber mixture;
10) A method for producing a fine carbon fiber comprising thermally decomposing a carbon material in the presence of a catalyst fluid containing a solvent and fine particles of a catalyst dispersed therein, wherein the fine particles have a size of 20 nm or less, and the catalyst comprises a transition metallic compound comprising at least one element selected from the group consisting of Fe, Ni, and Co;
11) A fine carbon fiber composition comprising a fine carbon fiber as recited in any one of 1) through 8) above;
12) A gas occlusion material comprising a fine carbon fiber composition as recited in 11) above; and
13) A secondary battery comprising an electrode material, wherein the electrode material is a fine carbon fiber composition as recited in 11) above.